Integrated circuit technology has revolutionized various fields, including computers, control systems, telecommunications, and imaging. In the field of imaging, the charge coupled device (CCD) has been made popular by its performance characteristics. Nevertheless, the solid state CCD integrated circuits needed for imaging are relatively difficult to manufacture, and therefore are expensive. In addition, because of the differing processes involved in the manufacture of the CCD integrated circuits relative to MOS integrated circuits, the signal processing portion of the imaging sensor has typically been located on a separate integrated chip. Thus a CCD imaging device includes at least two integrated circuits: one for the CCD sensor and one for the signal processing logic.
Another class of image sensors is the CMOS active pixel sensor. As noted in U.S. Pat. No. 5,625,210 to Lee et al. (“the '210 patent”), an active pixel sensor refers to an electronic image sensor with active devices, such as transistors, that are associated with each pixel. The active pixel sensor has the advantage of being able to incorporate both signal processing and sensing circuitry within the same integrated circuit because of the CMOS manufacturing techniques.
One popular active pixel sensor structure consists of four transistors and a pinned photodiode. The pinned photodiode has gained favor for its ability to have good color response for blue light, as well as advantages in dark current density and image lag. Reduction in dark current is accomplished by “pinning” the diode surface potential to the Pwell or Psubstrate (GND) through a P+ region.
In general, it is desirable to accumulate as much charge in the photodiode as possible to increase the signal level. This is generally referred to as having a large capacitance in the pixel cell. However, with an increased signal level (resulting from more accumulated charge), this may cause image lag due to the incomplete transfer of mobile charge from the diode to the floating output node, if the junction profile is not perfectly optimized for the charge transfer. This is also referred to as incomplete reset or incomplete depletion of the N-well of the pinned photodiode. A discussion of this phenomena can be found in “Characterization of Pixel Response Time and Image Lag in CMOS Sensors” by Ramaswami et al. The non-fully depleted state of the N-well is particularly evident with low voltage operation (for example 2.5 volts or lower). Low-voltage operation is becoming more and more prevalent as integrated circuit devices shrink and gate oxides become thinner.
Therefore, it is desirable to have an active pixel using a pinned photodiode that has high charge accumulation capabilities, yet complete reset of the photodiode even with low voltage operation. Another important consideration is to have a low leakage current for the pixel.